1. Field of the Invention
This application claims priority from U.S. Provisional application Ser. No. 60/082,521, filed Apr. 21, 1998. The present invention relates to new formulations of metal plating solutions designed to provide uniform coatings on substrates and to provide defect free filling of small features, e.g., micron scale features and smaller, formed on substrates.
2. Background of the Related Art
Electrodeposition of metals has recently been identified as a promising deposition technique in the manufacture of integrated circuits and flat panel displays. As a result, much effort is being focused in this area to design hardware and chemistry to achieve high quality films on substrates which are uniform across the area of the substrate and which can fill or conform to very small features.
Typically, the chemistry, i.e., the chemical formulations and conditions, used in conventional plating cells is designed to provide acceptable plating results when used in many different cell designs, on different plated parts and in numerous different applications. Cells which are not specifically designed to provide highly uniform current density (and the deposit thickness distribution) on specific plated parts require high conductivity solutions to be utilized to provide high xe2x80x9cthrowing powerxe2x80x9d (also referred to as high Wagner number) so that good coverage is achieved on all surfaces of the plated object. Typically, a supporting electrolyte, such as an acid or a base, or occasionally a conducting salt, is added to the plating solution to provide the high ionic conductivity to the plating solution necessary to achieve high xe2x80x9cthrowing powerxe2x80x9d. The supporting electrolyte does not participate in the electrode reactions, but is required in order to provide conformal coverage of the plating material over the surface of the object because it reduces the resistivity within the electrolyte, the higher resistivity that otherwise occurs being the cause of the non-uniformity in the current density. Even the addition of a small amount, e.g., 0.2 Molar, of an acid or a base will typically increase the electrolyte conductivity quite significantly (e.g., double the conductivity).
However, on objects such as semiconductor substrates that are resistive, e.g., metal seeded wafers, high conductivity of the plating solution negatively affects the uniformity of the deposited film. This is commonly referred to as the terminal effect and is described in a paper by Oscar Lanzi and Uziel Landau, xe2x80x9cTerminal Effect at a Resistive Electrode Under Tafel Kineticsxe2x80x9d, J. Electrochem. Soc. Vol. 137, No. 4 pp. 1139-1143, April 1990, which is incorporated herein by reference. This effect is due to the fact that the current is fed from contacts along the circumference of the part and must distribute itself across a resistive substrate. If the electrolyte conductivity is high, such as in the case where excess supporting electrolyte is present, it will be preferential for the current to pass into the solution within a narrow region close to the contact points rather than distribute itself evenly across the resistive surface, i.e., it will follow the most conductive path from terminal to solution. As a result, the deposit will be thicker close to the contact points. Therefore, a uniform deposition profile over the surface area of a resistive substrate is difficult to achieve.
Another problem encountered with conventional plating solutions is that the deposition process on small features is controlled by mass transport (diffusion) of the reactants to the feature and by the kinetics of the electrolytic reaction instead of by the magnitude of the electric field as is common on large features. In other words, the replenishment rate at which plating ions are provided to the surface of the object can limit the plating rate, irrespective of current. Essentially, if the current density dictates a plating rate that exceeds the local ion replenishment rate, the replenishment rate dictates the plating rate. Hence, highly conductive electrolyte solutions that provide conventional xe2x80x9cthrowing powerxe2x80x9d have little significance in obtaining good coverage and fill within very small features. In order to obtain good quality deposition, one must have high mass-transport rates and low depletion of the reactant concentration near or within the small features. However, in the presence of excess acid or base supporting electrolyte, (even a relatively small excess) the transport rates are diminished by approximately one half (or the concentration depletion is about doubled for the same current density). This will cause a reduction in the quality of the deposit and may lead to fill defects, particularly on small features.
It has been learned that diffusion is of significant importance in conformal plating and filling of small features. Diffusion of the metal ion to be plated is directly related to the concentration of the plated metal ion in the solution. A higher metal ion concentration results in a higher rate of diffusion of the metal into small features and in a higher metal ion concentration within the depletion layer (boundary layer) at the cathode surface, hence faster and better quality deposition may be achieved. In conventional plating applications, the maximum concentration of the metal ion achievable is typically limited by the solubility of its salt. If the supporting electrolyte, e.g., acid, base, or salt, contain a co-ion which provides a limited solubility product with the plated metal ion, the addition of a supporting electrolyte will limit the maximum achievable concentration of the metal ion. This phenomenon is called the common ion effect. For example, in copper plating applications, when it is desired to keep the concentration of copper ions very high, the addition of sulfuric acid will actually diminish the maximum possible concentration of copper ions. The common ion effect essentially requires that in a concentrated copper sulfate electrolyte, as the sulfuric acid (HS2O4) concentration increases (which gives rise to H+ cations and HSO4xe2x88x92 and SO4xe2x88x92 anions), the concentration of the copper (II) cations decreases due to the greater concentration of the other anions. Consequently, conventional plating solutions, which typically contain excess sulfuric acid, are limited in their maximal copper concentration and, hence, their ability to fill small features at high rates and without defects is limited.
Therefore, there is a need for new formulations of metal plating solutions designed particularly to provide good quality plating of small features, e.g., micron scale and smaller features, on substrates and to provide uniform coating and defect-free fill of such small features.
The present invention provides plating solutions with none or low supporting electrolyte, i.e., which include no acid, low acid, no base, or no conducting salts, and/or high metal ion, e.g., copper, concentration. Additionally, the plating solutions may contain small amounts of additives which enhance the plated film quality and performance by serving as brighteners, levelers, surfactants, grain refiners, stress reducers, etc.
The present invention generally relates to electroplating solutions having low conductivity, particularly those solutions containing no supporting electrolyte or low concentration of supporting electrolyte, i.e., essentially no acid or low acid (and where applicable, no or low base) concentration, essentially no or low conducting salts and high metal concentration to achieve good deposit uniformity across a resistive substrate and to provide good fill within very small features such as micron and sub-micron sized features and smaller. Additionally, additives are proposed which improve leveling, brightening and other properties of the resultant metal plated on substrates when used in electroplating solutions with no or low supporting electrolyte, e.g., no or low acid. The invention is described below in reference to plating of copper on substrates in the electronic industry. However, it is to be understood that low conductivity electroplating solutions, particularly those having low or complete absence of supporting electrolyte, can be used to deposit other metals on resistive substrates and has application in any field where plating can be used to advantage.
In one embodiment of the invention, aqueous copper plating solutions are employed which are comprised of copper sulfate, preferably from about 200 to about 350 grams per liter (g/l) of copper sulfate pentahydrate in water (H2O), and essentially no added sulfuric acid. The copper concentration is preferably greater than about 0.8 Molar.
In addition to copper sulfate, the invention contemplates copper salts other than copper sulfate, such as copper fluoborate, copper gluconate, copper sulfamate, copper sulfonate, copper pyrophosphate, copper chloride, copper cyanide and the like, all without (or with little) supporting electrolyte. Some of these copper salts offer higher solubility than copper sulfate and therefore may be advantageous.
The conventional copper plating electrolyte includes a relatively high sulfuric acid concentration (from about 45 g, of H2SO4 per L of H2O (0.45M) to about 110 g/L (1.12M)) which is provided to the solution to provide high conductivity to the electrolyte. The high conductivity is necessary to reduce the non-uniformity in the deposit thickness caused by the cell configuration and the differently shaped parts encountered in conventional electroplating cells. However, the present invention is directed primarily towards applications where the cell configuration has been specifically designed to provide a relatively uniform deposit thickness distribution on given parts. However, the substrate is resistive and imparts thickness non-uniformity to the deposited layer. Thus, among the causes of non-uniform plating, the resistive substrate effect may dominate and a highly conductive electrolyte, containing, e.g., high H2SO4 concentrations, is unnecessary. In fact, a highly conductive electrolyte (e.g., generated by a high sulfuric acid concentration) is detrimental to uniform plating because the resistive substrate effects are amplified by a highly conductive electrolyte. This is the consequence of the fact that the degree of uniformity of the current distribution, and the corresponding deposit thickness, is dependent on the ratio of the resistance to current flow within the electrolyte to the resistance of the substrate. The higher this ratio is, the lesser is the terminal effect and the more uniform is the deposit thickness distribution. Therefore, when uniformity is a primary concern, it is desirable to have a high resistance within the electrolyte. Since the electrolyte resistance is given by 1/xcexaxcfx80r2, it is advantageous to have as low a conductivity, xcexa, as possible, and also a large gap, 1, between the anode and the cathode. Also, clearly, as the substrate radius, r, becomes larger, such as when scaling up from 200 mm wafers to 300 mm wafers, the terminal effect will be much more severe (e.g., by a factor of 2.25). By eliminating the acid, the conductivity of the copper plating electrolyte typically drops from about 0.5 S/cm (0.5 ohmxe2x88x921 cmxe2x88x921) to about {fraction (1/10)} of this value, i.e., to about 0.05 S/cm, making the electrolyte ten times more resistive.
Also, a lower supporting electrolyte concentration (e.g., sulfuric acid concentration in copper plating) often permits the use of a higher metal ion (e.g., copper sulfate) concentration due to elimination of the common ion effect as explained above. Furthermore, in systems where a soluble copper anode is used, a lower added acid concentration (or preferably no acid added at all) minimizes harmful corrosion and material stability problems. Additionally, a pure or relatively pure copper anode can be used in this arrangement. Because some copper dissolution typically occurs in an acidic environment, copper anodes that are being used in conventional copper plating typically contain phosphorous. The phosphorous forms a film on the anode that protects it from excessive dissolution, but phosphorous traces will be found in the plating solution and also may be incorporated as a contaminant in the deposit. In applications using plating solutions with no acidic supporting electrolytes as described herein, the phosphorous content in the anode may, if needed, be reduced or eliminated. Also, for environmental considerations and ease of handling the solution, a non acidic electrolyte is preferred.
Another method for enhancing thickness uniformity includes applying a periodic current reversal. For this reversal process, it may be advantageous to have a more resistive solution (i.e., no supporting electrolyte) since this serves to focus the dissolution current at the extended features that one would want to preferentially dissolve.
In some specific applications, it may be beneficial to introduce small amounts of acid, base or salts into the plating solution. Examples of such benefits may be some specific adsorption of ions that may improve specific deposits, complexation, pH adjustment, solubility enhancement or reduction and the like. The invention also contemplates the addition of such acids, bases or salts into the electrolyte in amounts up to about 0.4 M.
A plating solution having a high copper concentration (i.e.,  greater than 0.8M) is beneficial to overcome mass transport limitations that are encountered when plating small features. In particular, because micron scale features with high aspect ratios typically allow only minimal or no electrolyte flow therein, the ionic transport relies solely on diffusion to deposit metal into these small features. A high copper concentration, preferably about 0.85 molar (M) or greater, in the electrolyte enhances the diffusion process and reduces or eliminates the mass transport limitations. The metal concentration required for the plating process depends on factors such as temperature and the acid concentration of the electrolyte. A preferred metal concentration is from about 0.8 to about 1.2 M.
The plating solutions of the present invention are typically used at current densities ranging from about 10 mA/cm2 to about 60 mA/cm2. Current densities as high as 100 mA/cm2 and as low as 5 mA/cm2 can also be employed under appropriate conditions. In plating conditions where a pulsed current or periodic reverse current is used, current densities in the range of about 5 mA/cm2 to about 400 mA/cm2 can be used periodically.
The operating temperatures of the plating solutions may range from about 0xc2x0 C. to about 95xc2x0 C. Preferably, the solutions range in temperature from about 20xc2x0 C. to about 50xc2x0 C.
The plating solutions of the invention also preferably contain halide ions, such as chloride ions, bromide, fluoride, iodide, chlorate or perchlorate ions typically in amounts less than about 0.5 g/l. However, this invention also contemplates the use of copper plating solutions without chloride or other halide ions.
In addition to the constituents described above, the plating solutions may contain various additives that are introduced typically in small (ppm range) amounts. The additives typically improve the thickness distribution (levelers), the reflectivity of the plated film (brighteners), its grain size (grain refiners), stress (stress reducers), adhesion and wetting of the part by the plating solution (wetting agents) and other process and film properties. The invention also contemplates the use of additives to produce asymmetrical anodic transfer coefficient (xcex1a) and cathodic transfer coefficient (xcex1c) to enhance filling of the high aspect ratio features during a periodic reverse plating cycle.
The additives practiced in most of our formulations constitute small amounts (ppm level) from one or more of the following groups of chemicals:
1. Ethers and polyethers including polyalkylene glycols
2. Organic sulfur compounds and their corresponding salts and polyelectrolyte derivatives thereof.
3. Organic nitrogen compounds and their corresponding salts and polyelectrolyte derivatives thereof.
4. Polar heterocycles
5. A halide ion, e.g., Clxe2x88x92
Further understanding of the present invention will be had with reference to the following examples which are set forth herein for purposes of illustration but not limitation.